To facilitate data communication over an international network, a standard has been promulgated to define the communication protocol for use in optical digital networks. This standard is discussed in detail in International Telecommunication Union, CCITT, General Aspects Of Digital Transmission Systems Recommendations G.707-G.709 (Geneva 1989), which is incorporated herein by reference. The CCITT standard known as the Synchronous Data Hierarchy ("SDH" or "the standard") defines a hierarchy of data containers and their pointers for standardized high bit rate transmission. Under the CCITT hierarchy, data signals are, in general, mapped into containers and then transmitted in transport module frames. Recommendation G.708 .sctn.2.2. The SDH defines several types of standard data containers, thereby allowing SDH to accommodate data signals operating at various standard bit rates. See, for example, CCITT FIG. 1-1/G.709.
Under one implementation of the standard that is experiencing widespread use, sixty-three 2 Mbit/s data signals are mapped into VC-12 containers, multiplexed and transmitted using transport module frames. The multiplexed VC-12 containers are first arranged into larger data containers known as C-4 containers. Each C-4 container is designed to fit into one transport module frame known as an STM-1 frame. Because of the respective sizes of the C-4 and VC-12, containers, sixty-three multiplexed VC-12 containers do not fit within one C-4 container and must be split over four consecutive C-4 containers, and therefore, four consecutive STM-1 frames. The four frame signal sequence is known as a multiframe signal.
Under the SDH standard, a C-4 container is converted into a VC-4 container before it is placed into an STM-1 frame. The VC-4 adds critical overhead data, including, in particular, data indicating the VC-4 container's position within the multiframe signal sequence. With this multiframe position data, referred to as multiframe synchronization (MFS) information, the terminal equipment at a node within the network receiving STM-1 signals can determine the beginning and end of a multiframe signal. Without the MFS information, the VC-12 data within the STM-1 signals cannot be accessed for reasons that will become clear hereinafter. The standard C-4 container is equivalent to a VC-4 in most respects, with the exception that the VC-4 also contains path overhead information.
The digital optical network that transports the SDH-formatted signals includes various nodes that provide maintenance, trafficking and error checking functions. One such node cross-connects data that is received as STM-1 frames on the VC-12 level. This node, called a cross connect node, rebuilds new STM-1 cells after cross-connection for transmission over the system. The node cross connect circuitry, however, does not cross-connect data directly from the STM-1 frames. Instead, the node circuitry must process the STM-1 frame to reproduce the data in the form of C-4 containers, which are simply multiplexed columns of VC-12 data. The presentation of data in C-4 container form facilitates cross-connecting on the VC-12 level.
Moreover, the node containing the cross connect circuitry does not receive data exclusively in the STM-1 format. In addition, the node may receive lower order signals, including, for example, plesiochronous 2 Mbit/s signals (CCITT standard), that must be mapped to VC-12 containers and multiplexed into standard container format. In either case, the cross-connecting circuitry is designed to handle data that has been converted to the standard C-4 container format.
The physical structure of the cross connect node may comprise several physical component racks or subracks in order to accommodate several input/output ports. In order to allow for flexibility in the physical configuration of the several subracks, the system must be capable of transmitting data considerable distances between subracks. Such flexibility necessitates intranodal or intrasystem communication over distances that may well be in excess of 10 meters. The transmission of standard containers or C-4 containers within the node at sufficiently high bit rates requires the use of communication links. Such intrasystem communication requires a communications protocol to transfer data between the input/output racks and the cross connect core circuitry.
One way to facilitate intrasystem communications is to transport the standard containers using the SDH standard STM-1 frame. Such an approach is logical considering that the standard container is already structured for use in this context. However, the SDH requires that each standard container, in other words, each group of multiplexed VC-12 containers, must first be mapped into a VC-4 container, which in turn requires the generation of a path overhead. See, for example, CCITT Recommendation FIG. 5-1/G.708. Because generation of the VC-4 and its path overhead requires additional hardware in the intrasystem link, it provides a less than optimal solution.
An alternative method is to employ a separate communications protocol to deliver the standard container intact. The implementation of a new protocol, however, would likely incur significant development costs and delays. Because such costs and delays would be incurred and a new protocol would presumably introduce non-standard codes into the data, this solution is also less than optimal.
It is therefore an object of the invention to provide a method and apparatus of transporting multiframe signals of VC-12 data in standard container form within a network node without requiring unnecessary hardware or significant development costs.